ESS15 Lecture 7 The Greenhouse effect.
Housekeeping. First midterm is in one week. Open book, open notes. Covers material through end of Friday s lecture Including today s lecture (greenhouse effect) And Friday s (energy balance of the Earth).
Housekeeping. Stay tuned for study guide, practice questions, and printable versions of lecture slides. Monday s lecture will be a review. Don t forget, I have office hours: Today 3:30-4pm, Monday 3-4pm.
Laboratory demonstration.
Review
i-clicker survey: Shorter wavelength radiation carries energy compared to longwave radiation and moves. A: more, faster B: less, shorter C: more, at the same speed D: more, shorter E: less, at the same speed
Electromagnetic Radiation Radiation travels as both waves and photons Waves do not require molecules to propagate Shorter waves have more energy than longer waves Waves interact with matter at similar scales as the waves wavelength wavenumber speed of light frequency
Diagram of the solar radiation budget 30% reflected by clouds, air, dust, and surface 19% absorbed by the atmosphere (mostly clouds) 51% absorbed at the surface
Blackbody Radiation SUN Stefan-Boltzmann Law: F = σt 4 EARTH σ = 5.67 x 10-8 Wm -2 K -4 Hot objects emit much more energy than cold objects at all wavelengths Peak emission at shorter waves at higher temperatures
The power of math: If we know..solar constant, Watts in = Watts out = and the albedo We can solve for a planet s temperature! This is the bare rock model: A climate prediction from laws of energy balance, black body radiation and geometry!
What temperature does the bare rock model predict? Minus eighteen degrees Celsius. -18C!!!?? If Earth were this cold it would: have solid frozen oceans and miles of ice. blindingly bright So something important must be missing from the model The atmosphere! The Earth is not a bare rock. If it were it would be real cold here on the surface. But the atmosphere blankets our rocky surface. This makes a big difference to the temperature.
The Greenhouse Effect.
Energy Balance with Atmosphere Sun beams down 51 units of shortwave energy WOW: 96 more units of longwave energy are beamed down from atmospheric gases and clouds! Earth's surface loses these 147 units through convection, evaporation, and radiation emission (glowing)
i-clicker exercise: of the energy beaming down to heat the surface of the Earth comes from air molecules. A: 1/3 B: 1/2 C: 2/3 D: 3/4
Greenhouse Gases
National Academies: Greenhouse Gases: Climate Change https://www.youtube.com/watch?v=3jx-iosmnw8&index=3&list=pl38eb9c0bc54a9ee2
Greenhouse Gases have been known for over 150 years
John Tyndall (1820-1893)
Laboratory apparatus in the era of steam-engines.
Measuring how infrared radiation interacts with different types of gases. IR Absorp;on
Without water vapor, the Earth s surface would be held fast in the iron grip of frost.
http://changingclimates.colostate.edu/movies/scott_denning_796kbits.mov How do Greenhouse Gases Absorb Energy?
Atmospheric Gases Gases are made of moving molecules separated by empty space Kinetic energy of molecular motion is proportional to temperature Gases don t behave as blackbodies or even graybodies!
I-Clicker Survey a. CH 4 (Methane) b. N 2 (Nitrogen) c. O 2 (Oxygen) d. CO 2 (Carbon Dioxide) What is the most abundant gas in Earth s atmosphere?
Waves and Photons Electromagne,c radia,on behaves as both waves and par,cles! Long Waves = small photons Short Waves = BIG PHOTONS Experiments show both kinds of behavior: Oscillating fields with troughs and crests (waves) Individual packets of energy (particles)
When radiation meets matter.
Energy is Quantized When radiation interacts with atoms and molecules, only certain jumps in energy are possible Electrons orbit at specific energy levels above an atomic nucleus Absorption of a photon of just the right energy can make them jump up to the next level Emission of a photon occurs when an electron falls down to a level below
Atomic Emission Big jumps take lots of energy, small jumps take less Big drops emit energetic photons (short wavelengths) Small drops emit less energetic photons (longer wavelengths) Most electron transitions in gases absorb/emit visible or UV light
Molecules and Photons Molecules are groups of atoms that share electrons (bonds) Molecular transitions involve changes in vibration, rotation, bending, and stretching of chemical bonds Photons can interact with molecules to change states Transitions involve specific amounts of energy, so only certain wavelengths are active Molecular transitions typically absorb and emit in thermal infrared
Dancing molecules and heat rays Nearly all of the air is made of oxygen (O 2 ) and nitrogen (N 2 ) in which two atoms of the same element share electrons O O N N Diatomic molecules can vibrate back and forth like balls on a spring, but the ends are identical Infrared (heat) energy radiated up from the surface can be absorbed by these molecules, but not very well
Dancing molecules and heat rays Carbon dioxide (CO2) and water vapor (H2O) are very different! O C O H O H Molecules that have many ways to wiggle are called greenhouse molecules. They have many more ways to vibrate and rotate, so they are very good at absorbing and emitting infrared (heat) radiation